In recent years, the studies of an organic thin-film LED (LED: light emitting device) have actively been conducted such that an electron injected from a cathode and a hole injected from an anode recombine in an organic compound held between both of the cathode and the anode. This LED is characterized by thin type, high brightness under low drive voltages and multicolor light emission through selection of light-emitting materials, thereby attracting notice.
These studies have been conducted by many research institutions since it was revealed by C. W. Tang et al. of Kodak Company that an organic thin-film LED emitted light with high brightness. The typical structure of an organic thin-film LED presented by a study group of the Kodak Company is such that a diamine compound for hole transporting, tris(8-quinolinolate)aluminum (III) as a luminescent layer and Mg:Ag as a cathode are sequentially layered on an ITO glass substrate, which LED allows green light emission of 1000 cd/m2 at a driving voltage of approximately 10 V (refer to Nonpatent Literature 1).
The use of various fluorescent materials for a luminescent layer allows an organic thin-film LED to obtain diverse luminescent colors, so that the studies of practical application to displays are actively carried out. Among light-emitting materials of the three primary colors, the studies of green light-emitting materials are the most advanced, and the earnest studies in red light-emitting materials and blue light-emitting materials are presently made toward performance improvement.
One of the largest problems in an organic thin-film LED is to improve durability of the device. In particular, with regard to blue, few blue light-emitting materials provide a device having excellent durability and high reliability. For example, a technique is disclosed in which a styrylamine derivative (refer to Patent Literature 1) and a perylene derivative (refer to Patent Literature 2) are used as a blue dopant material. Also, a technique is disclosed in which a pyrene compound is used for a blue light-emitting device. Blue light-emitting devices using various pyrene compounds (refer to Patent Literatures 3 to 5) are reported; however, any of them has insufficient durability.
(Nonpatent Literature 1)
Applied Physics Letters (USA) 1987, Vol. 51, No. 12, pages 913 to 915
(Patent Literature 1)
Japanese Unexamined Patent Publication No. 5-17765 (claim 1)
(Patent Literature 2)
Japanese Unexamined Patent Publication No. 2003-86380 (claim 3)
(Patent Literature 3)
Japanese Unexamined Patent Publication No. 2001-118682 (claim 1)
(Patent Literature 4)
Japanese Unexamined Patent Publication No. 2004-75567 (claims 1 to 4)
(Patent Literature 5)
International Publication No. 2004/096945 Pamphlet (Claims)